The present invention relates to an electronic musical instrument, and in particular to a waveform synthesizing circuit for both producing the synthesized waveform and keying it with the desired envelope.
At the present time, drum sounds, such as bass drums, congas, and the like, are simulated in electronic organs by driving a ringing oscillator with a pulse input. A ringing oscillator is basically a high Q filter or underdamped oscillator, and when it is excited by a pulse input, it rings or oscillates for a brief period of time. The output of this type of circuit is essentially a damped sinusoid waveform having a relatively fast attack and a more gradual decay and having a duration appropriate to the particular rhythm instrument being simulated.
The problem with this type of circuit is that if the Q is set too high, the circuit will become less damped and will tend to oscillate. The sound produced by circuits of this type sound more pleasing when the Q is set as high as possible, and as long as the circuit elements remain within tolerance and are not subjected to excessive temperature conditions, the circuit will perform satisfactorily. As the circuit element tolerance changes and temperature conditions vary, however, the circuit can drift into an oscillatory condition, which produces a very unpleasing howling or humming sound. In order to avoid this condition, most damped sine wave circuits of this type are set to be damped more than is desirable to obtain an optimum sound and the simulation of the percussion instruments in question is correspondingly affected. A further problem is that the circuits are generally made of discrete components which vary from unit to unit and the sound character produced by the units will not be uniform.
Another prior art circuit employed to produce certain percussion sounds comprises generating a square wave in synchronism with a pulse from the rhythm unit counter or ROM output, and a square wave signal is then appropriately filtered to produce a damped sine wave. Difficulty with this type of circuit is that, depending on when the leading edge of the pulse from the rhythm unit occurs in relation to the square wave pulse train, a different sound will be produced. The differences in resultant sound occur primarily to differences in the attack portion of the waveform, which is a function of the relationship between the rhythm pulse and the square wave pulse train.
Although the particular embodiment of the present invention disclosed hereinafter is a circuit for producing a damped sine wave suitable for use in the production of certain drum and other percussion instrument sounds, the invention also relates to other types of waveforms, such as reeds, violins and the like.
Typically, the tones produced by an electronic musical instrument are initially generated by a tone generator, which produces a plurality of tones spanning the entire range of the musical instrument. These tones are then connected to the tone inputs of individual keyers that have control inputs adapted to receive keying signals initiated by depressing keys of the keyboard or from an automatic easy play feature, such as a musical rhythm accompaniment system. The keying signals are often in the form of long duration pulses that are appropriately filtered to impart exponential-type attack and decay leading and trailing portions. The keying signals directly control the amplitude of the tones keyed by the keyers, and by varying the attack and decay characteristics, a wide variety of percussive and sustain-type instruments can be simulated.
A second type of tone generation and keying, which is commonly referred to as digital tone synthesis, comprises storing in a memory a digital representation of the actual waveshape. The digitized and stored waveshape is then read out repetitiously at a particular frequency and converted to analog form to produce the musical note, with the rate at which the waveshape is read out determining the frequency of the resultant tone. Attack and decay are controlled digitally, as by the addition and subtraction of scaling factors.
Both in the keyed oscillator and digital synthesis techniques for tone generation, it is necessary to key the tone to the output with an attack and decay appropriate to the particular instrument being simulated. Although the keying in the digital synthesis technique is usually handled by a pure digital technique, the keyed oscillator systems have often relied on percussive and sustain envelope generators wherein external RC timing circuits are employed. To avoid the necessity for external discrete elements, switched capacitor techniques have been used to produce the keying envelope wherein a pair of alternately switched electronic switches having a capacitor connected to their juncture incrementally transfer the voltage from the input to the output. The advantage to the switched capacitor technique is that it can be fully integrated thereby avoiding the necessity for external timing capacitors. The use of such a circuit in a sustain-type keyer is disclosed in U.S. Pat. No. 4,205,581, and a percussion-type keyer utilizing this technique is disclosed in U.S. Pat. No. 4,205,582. Each of these two patents is expressly incorporated herein by reference.